We had two projects to investigate the role of CA3 in the pathology of CA1 excitotoxic cell death and of chronic stress, using CA3-NR1 KO mutants in which the essential NR1 subunit (GluN1) of the NMDA receptor is selectively eliminated from CA3 pyramidal neurons (Nakazawa et al., 2002). First, we hoped to determine what mechanisms could lead from CA3 NR1 ablation to CA1 excitotoxicity. CA1 is one of the most vulnerable areas to excitotoxic cell death in the brain, and CA3 is an intermediate relay site of hippocampal circuitry and thus positioned to play a role in gating activity levels in CA1. Indeed, we found that ablation of CA3 NR1 in mice results in an increase in kainite-induced seizure intensity and in selective CA1 cell death. Secondly, we sought to test the hypothesis that the CA3 NMDA receptor was critical for mediating the chronic stress-induced structural changes observed in CA3 (one of the brain areas most vulnerable to chronic stress) as well as to assess the impact of altered CA3 dendritic remodeling on behavior. #1 Lack of kainic acid-induced gamma oscillations predicts CA1 excitotoxic cell death The observation that CA3-NR1 KO mutants showed abnormally high sharp wave-ripple activity in the hippocampal CA1 region during periods of awake immobility (Jinde et al., 2009) led us to predict that these mutants are susceptible to epileptic insults. Indeed, kainic acid (KA) i.p. injection in these mutants induced severe behavioral seizures and marked degeneration of both CA1 pyramidal cells and GABAergic neurons. This pattern was not observed in control genotypes. To investigate what mechanisms in the mutants could contribute to KA-induced vulnerability of seizures and neurodegeneration, we recorded in vivo neural activity in CA1 before and after KA treatment and observed KA-induced persistent 30-50 Hz gamma oscillations in control mice prior to the first seizure discharge that were absent in the mutants (Jinde et al., 2009). Consequently, on subsequent days, mutants manifested prolonged epileptiform activity and massive cell death of both pyramidal cells and local GABAergic interneurons in CA1. Remarkably, pretreatment with alpha dendrotoxin, which is known to enhance the presynaptic GABA release, maintained the KA-evoked gamma oscillations, diminished epileptiform activity, and prevented CA1 cell death in the mutants. This result indicates a crucial role for CA3 neurons in modulating the inhibitory network within the hippocampus and controlling levels of excitability in pathological conditions. In particular, the present results demonstrated that KA-induced gamma oscillations are negatively correlated with subsequent CA1 cell death. Considering that the emergent gamma oscillations are frequently observed in human epileptic patients, especially before and at the onset of seizure discharges, our results demonstrate the predictive role of gamma oscillations in limbic neurodegeneration prior to seizures in some epilepsy patients. #2 Chronic stress-induced dendritic retraction requires CA3 NMDA receptors Chronic stress leads to the development of a number of neurostructural, neuroendocrine and behavioral changes that may also precipitate the onset of many neuropsychiatric disorders. In animal models, one of the most dramatic effects of chronic stress in the brain is dendritic remodeling in the CA3 region of the hippocampus. It has remained unclear whether this structural change to the input region of CA3 neurons plays a causal role in the development of cognitive deficits associated with stress-related disorders or is an adaptive response that serves to minimize further damage to the hippocampal circuit. First, we found that stress-induced dendritic retraction of both dorsal and ventral CA3 short-shaft pyramidal neurons, but not long-shaft pyramidal neurons, does require a functional NMDA receptor. CA3-NR1-KO mice were impervious to the effects of chronic immobilization stress, showing no structural changes as were observed in control mice. Further, this CA3-specific genetic manipulation also prevented stress-induced atrophy in CA1, demonstrating a feed-forward effect of CA3 atrophy on the CA1 subregion of the hippocampus. However, lack of dendritic retraction of the hippocampal pyramidal neurons in the mutant mice had no effect on HPA axis activation, evidenced by the blood corticostrerone increase and the atrophy of adrenal gland, and on the levels of anxiety and locomotion that were induced by chronic stress. More importantly, the genetic manipulation had almost no impact on the spatial short-term memory and contextual long-term fear memory. In other words, hippocampal remodeling was uncoupled from stress-induced physiology and behavioral performance. Thus, the present results suggest that hippocampal atrophy per se may not be sufficient to explain stress-induced physiological and behavioral changes (Christian et al., Chronic stress-induced hippocampal dendritic retraction requires CA3 NMDA receptors. manuscript under review).